Long microwave-photonic variable delay of linear frequency modulated waveforms

The long, microwave-photonic variable delay of the impulse response of linear frequency-modulated (LFM) waveforms is proposed and demonstrated experimentally. The delay method is based on the application of a variable frequency offset to one side-band of an LFM-modulated optical carrier. Due to the time-frequency ambiguity properties of the LFM signal, the effect of the frequency offset on the impulse response is nearly equivalent to that of a group delay. Numerical simulations suggest negligible degradations in the resolution, peak-to-side-lobe ratio and integrated side-lobe ratio of the processed LFM waveform. Experiments demonstrate the delay of a 500-MHz-wide LFM signal by up to 250 ns. The method is applicable to LFM waveforms of arbitrary bandwidth and central radio-frequency. The long delays make the method attractive for optical beam-steering in large phased-array radars.